1. Field of the Invention
This invention relates to industrial high relative viscosity (RV) filaments, such as, for use in papermaking machine felts, apparatus and processes for solid phase polymerizing polyamide flake suitable for use in making the filaments, and processes for making the filaments.
2. Description of Related Art
Industrial polyamide filaments are used in, among other things, tire cords, airbags, netting, ropes, conveyor belt cloth, felts, filters, fishing lines, and industrial cloth and tarps. When used as staple fibers for papermaking machine felts, the fibers must have generally good resistance to chemicals and generally good wear resistance (e.g., resistance to abrasion, impact and flex fatigue). Such felts are often exposed to oxidizing aqueous solutions which can seriously shorten the service life of the felt.
Stabilizers are often added to polyamides for the purpose of increasing chemical resistance. The amount of stabilizer which can be introduced is limited, however, due to excess foaming that occurs during polymerization when stabilizers are added to autoclaves or continuous polymerizers (CPs).
It is also desirable to spin filaments which have a high RV to improve resistance to chemicals and to wear from abrasion, impact and flexing. However, in the past, when the polyamide supply for such filaments is polyamide flake, it was often difficult, if not impossible, to obtain the desired high RV while maintaining polymer quality, e.g., low level of cross linking and/or branching.
One way to increase the RV is to increase the amount of catalyst during polymerization in an autoclave, continuous polymerizer (CP), or elsewhere in the process, but this causes process and/or product problems. Difficulties, for instance, similar to those encountered with stabilizers can occur when catalysts are added in high quantities. Further, high quantities of catalysts in the autoclave can cause severe injection port pluggage and complications to injection timings during autoclave cycles. High quantities of catalysts injected into CPs place stringent demands on equipment capability because of high levels of water loading.
In U.S. Pat. No. 5,236,652, Kidder discloses such a process for making polyamide fibers for use as staple for papermaking machine felt. This process comprises (i) melt-blending polyamide flake with a polyamide additive concentrate which is made of a polyamide flake and an additive selected from the group of stabilizers, catalysts and mixtures thereof, and (ii) extruding the melt-blended mixture from a spinneret to form the higher RV fibers. Processes that add catalyst concentrate to polyamide flake, like the Kidder process, require special feed apparatus for metering the concentrate to the flake which significantly increases the expense of operating such a process. Further, adding high concentrations of catalyst to the polyamide often results in process and/or product control difficulties. Cross linking and/or branching of the fiber, and more susceptibility to chemical attack are liabilities of using high catalyst levels in polyamides.
Another way to increase the RV is through solid phase polymerization (SPP) of the polymer. In U.S. Pat. No. 5,234,644, Schutze et al. disclose a post spin SPP process for making high RV polyamide fibers for use in paper machinery webs. In this case, in contrast to prior staple fiber manufacturing processes, the post spin SPP process requires an added step after spinning the fibers with special processing equipment to increase the RV of the fibers. This special equipment adds a significant cost to the producer and the added post spinning step takes additional time to make the fibers. Furthermore, uniform fiber property control is more difficult when the post spinning SPP step is performed in a batch mode.
Thus, there is a long felt need for filaments with higher RV polyamide than previously made, and apparatus and processes for making the filaments for industrial uses, such as, in making papermaking machine felts, without process and product problems, such as those described above.
These and other objects of the invention will be clear from the following description.